LLC converters are most efficient when operating at their resonant frequency. Small deviations in the switching frequency of an LLC converter can have significant impacts on its efficiency. Consequently, maintaining operation of an LLC converter at, or close to, its resonant frequency is important in maintaining an efficient system. The resonant frequency of an LLC converter is dependent upon the components of the LLC converter (e.g., the inductors, capacitors, etc.). Due to manufacturing tolerances, LLC converters having the same components may have different resonant frequencies. That is, the components may vary from LLC converter to LLC converter resulting in different resonant frequencies for each LLC converter. For example, the manufacturing tolerance for an inductor may be 20%. Consequently, even though the same inductor is used in each LLC converter, the inductance of the inductors may vary from LLC converter to LLC converter. Because of these manufacturing tolerances, the resonant frequency can vary from LLC converter to LLC converter.
While operating an LLC converter at a switching frequency higher than the resonant frequency results in inefficiencies, operating an LLC converter at a switching frequency lower than the resonant frequency can result not only in inefficient operation but also damage to the LLC converter. Specifically, if the on-time of the switches on the secondary side of the LLC converter is too long, current can flow in a backwards direction in the LLC converter. Because damage can result to the LLC converter if the on-time of switches on the secondary side is too long (i.e., the switching frequency is shorter than that required by the resonant frequency), LLC converters are configured to utilize a conservative maximum on-time. That is, regardless of the qualities of the components used in each LLC converter, each LLC converter will have the same maximum on-time for the switches on the secondary side to ensure that the on-time of the secondary side switches is not so long as to risk damaging the LLC converter.
While this conservative on-time can prevent damage to the LLC converter, it introduces potential inefficiencies. For example, if the components of a specific LLC converter result in a longer resonant period than the conservative maximum on-time estimate, the on-time of the switches on the secondary side is too short resulting in diode conduction time and power losses (i.e., inefficiencies). Consequently, a need exists for a circuit that is capable of measuring body diode conduction time, adjusting the maximum on-time of the secondary side switches, and calculating the resonant frequency to optimize the LLC converter efficiency.